Semiconductor and the associated technologies have made remarkable progress in recent years. Concomitantly, the degree of integration of large scale integration (LSI) has more and more increased, resulting in a rapid progress of miniaturization of wiring and enlargement of tip size. With increase in the degree of integration of LSI memory, packaging methods are shifting over from through-hole packagings to surface packagings. That is, packaging methods have been shifting over from conventional dip-type packagings to surface packagings such as small-sized and thin-type flat packagings, small outline packages, J Bend Soic, plastic leaded chip carriers and the like.
As the result, problems such as package cracking and impairment of moisture resistance of the package due to the package cracking are becoming an issue. Especially, a rapid change in temperature of a package during the soldering operation of lead wires at the surface packaging step creates many troubles such as package cracking.
Epoxy resin curing agents commonly used in sealing compounds for semiconductors are phenol-type novolac resins or cresol-type novolac resins, said compounds being compounded with the same resins.
The main drawback of the epoxy resin compositions containing said curing agents is their susceptibility to cracking and the lack of moisture resistance. Because of this, such compounds can not follow effectively the rapid progress of the recent semiconductor technology.
To surmount the drawback, Japanese Provisional Publication No. 110213-63 discloses a moisture and thermal resistance phenolic resin, for example, a dicyclopentadiene derivative of a phenolic resin useful as epoxy resin curing agents. However, one of the disadvantages of the dicyclopentadiene derivative of phenolic resin is its poor moldability, and only at most, the dimer and trimer of cyclopentadiene may be used because the softening point of the derivative should be suppressed as much as possible. As a result, the glass transition point (Tg) of the resulting sealing compound is difficult to raise, and the electrical properties of the compound can not be improved.
Several processes for preparing a phenolic resin derivative by use of a low molecular weight butadiene polymer or copolymer as a starting material have been proposed. U.S. Pat. No. 3,258,450 discloses a process comprising addition of phenol to a low molecular butadiene polymer or copolymer in the presence of an activated clay or sulfuric acid as a catalyst. The problem of this process is that only addition polymers or copolymers having a softening point less than 22.degree. C. may be obtained. U.K. No. 1,106,267 also discloses a process comprising addition of phenol to the double bonds of a low molecular weight butadiene polymer or copolymer in the presence of phosphoric acid as a catalyst. Although the resulting resin has a relatively high softening point, the main disadvantage of the phenolic resin is the carbon-carbon double bonds remain in the molecule in a large number, and thus when it is used as a curing agent for sealing compounds, the long-term storage stability impaired.
A process comprising addition of phenol to the double bonds of a low molecular butadiene polymer or copolymer in the presence of perchloric acid as a catalyst has been proposed [Angew. Makromol. Chem., 24, 205 (1972)]. The problems of the process are also the remaining carbon-carbon double bonds as well as the side reactions of phenol which produce various higher molecular weight butadiene polymers and ether-type adduct other than the desired phenol adduct. Therefore, the phenolic resins obtained by the process are also not suitable for use as a resin for sealing compounds and the like.
Further, Japanese Provisional Publication No. 26894-54 describes a process for preparing a phenol adduct of polybutadiene comprising an installment addition of a low molecular weight butadiene polymer or copolymer to a mixture of phenol and BF.sub.3 ether complex. In such case, although higher molecular weight butadiene polymers may be suppressed, a large number of carbon-carbon double bonds may remain in the molecule of the resulting adduct. Furthermore, a process for preparing a phenol adduct of polybutadiene has been known wherein the process comprises reacting phenol with a low molecular weight butadiene polymer or copolymer in the presence of BF.sub.3 phenol complex as a catalyst (Japanese Provisional Publication No. 160453-54). The problems of this process are the unpreferable properties of the product having a softening point less than 75.degree. C., and the large number of remaining double bonds.